Planet 9 won’t be as big as Uranus. Really.

This week’s news in the Astronomical Journal that Mike Brown and Konstantin Batygin of CalTech have predicted a ninth planet got the usual barrage of silly media headlines about ‘discovery’.

The putative Planet 9 and the orbits of the KBOs that were used to deduce the possibility. Credit: Caltech/R. Hurt (IPAC); [Diagram created using WorldWide Telescope.] - See more at:
The putative Planet 9 and the orbits of the KBOs that were used to deduce the possibility. Credit: Caltech/R. Hurt (IPAC); [Diagram created using WorldWide Telescope.] – See more at:
The thing is, Brown and Batygin haven’t found anything. They’ve hypothesised that there’s a planet. And they are not the first to wonder about some kind of outer-outer planet – there’s been speculation for decades that an Earth-plus size world might be lurking in the dim reaches of the outer solar system. The problem is that it wasn’t found last time gravity-pertubation calculations were made. Or the time before that. Or the time before the time before that. Or… yah, you get the picture.

What makes this hunt different, though, is that Brown is a planet-finder of long experience – among other discoveries, he found Eris (nee Xena) which triggered the IAU’s 2006 re-think about what defines ‘planet’. So what’s their hypothesis? By analysing the orbits of six Kuiper Belt objects – including the dwarf planet Sedna – which probably wouldn’t be where they are if something hadn’t shepherded them, Brown and Batygin postulate a planet, between four and ten times the mass of Earth in an inclined and egg-shaped orbit far beyond Pluto and the Kuiper Belt. To put that in perspective, the upper mass limit is almost as much as Uranus.

The principle is sound for a variety of reasons. That ‘discovery by calculation’ is how Neptune was found. In 1821, Alexis Bouvard published data showing where Uranus should be – but the planet failed to co-operate. The wobble in Uranus’ orbit was put down to the tug of an unknown planet. British mathematician John Couch Adams got to work in the early 1840s figuring out where that unknown planet might be. So did French mathematician Urbain le Verrier. The first search at the urging of Astronomer Royal Sir George Airy, in June 1846, failed to find anything. But then in September Johann Galle of the Berlin Observatory found the new world where le Verrier had predicted.

For all that, the perturbation discovery method doesn’t always work. One of the reasons is that the so-called ‘n-body’ equation can’t be solved. The maths only work for two bodies, not more (mathematically, n = 2 is fine, but n = >2 isn’t). That means the answers have to be modelled (approximated). And sometimes, planets move oddly for other reasons. In the nineteenth century there was speculation that specific shifts (precession) in Mercury’s perehelion were caused by the tug of an unknown world orbiting closer to the Sun and invisible to us against the Sun’s glare. That was disproven by Albert Einstein, whose theory of General Relativity accounted for Mercury’s precession without recourse to another planet. Mercury was exactly where Einstein said it should be, which was one of the first proofs of his theory.

Let me illustrate mass vs surface gravity. Although it has a mass 14.5 times that of Earth, 'surface gravity' on Uranus is just 89 percent that of Earth. That's because the radius is about 4 times Earth's. I made this picture with Celestia.
Although it has a mass 14.5 times that of Earth, ‘surface gravity’ on Uranus is just 89 percent that of Earth. That’s because the radius is about 4 times Earth’s. I made this picture with Celestia.

In the outer solar system, pertubation analysis still applies – though as I say, prior studies haven’t added up to a discovery. The current one hasn’t, yet, either – though the search hasn’t got going in earnest. On the plus side, Brown and Batygin, with colleague Hayden Betts, earlier modelled how the main planets ended up where they are. By that 2012 hypothesis, to get the current setup out of the chaotic bash-or-flick pool game of the solar system’s formation, a fifth giant planet about the mass of Uranus (or a little less) was flung either out of the solar system altogether, or into a distant orbit.

If anything’s orbiting where Brown and Batygin propose, it’ll be incredibly dim despite its size – which explains why it hasn’t been found by accident so far. If it turns up, thanks to the criteria now used to define ‘planet’ – which knocked Pluto off its perch in 2006 – it would be Planet Nine.  And it’ll have come from outer space. (Geddit? Geddit? Not as lame as endless ‘Uranus’ jokes, is it…)

What’s next? Well, proof will only come with pictures. That won’t be easy. Japan’s 8.2-metre Subaru telescope, at Mauna Kea, has been tipped for the search – and can likely pick up the putative planet even if it’s at aphelion, its furthest point from the Sun. But there will be issues of ‘telescope time’ (a new planet has to be imaged at least twice, at sufficient interval to show it moving against the background stars, which is longer the further out you go; and that search also isn’t the only priority). Although Subaru’s field of view is wa-a-a-a-ay better than other large telescopes, it’s still only o.5 degrees of sky at a time, which is about the diameter of the full Moon seen from Earth. Nobody knows exactly where on its possible orbit Planet 9 might be, so there’s a relatively large area to survey. That, I expect, is why the estimate I saw yesterday put the likely search time at up to five years.

The other question is what the new world will be called, if it exists. That’s up to the IAU, the body responsible for naming them. Oceanus, maybe? That was first proposed for Neptune. Certainly they should avoid a name vulnerable to weak scatological humour, like Uranus. Or maybe they shouldn’t. It could be quite amusing.

Copyright © Matthew Wright 2016

13 thoughts on “Planet 9 won’t be as big as Uranus. Really.

  1. It would certainly be neat to find a new planet. I can dimly remember how exciting it was when Charon was found around Pluto, and it’d be wonderful to see something of that magnitude. But there’ve been so many putative new planet discoveries, and so far, there’s such a slender thread of evidence behind this one. Hard to get excited just yet.

    1. I remember when Charon was announced – stunning bit of news. I agree that the Planet 9 ‘discovery’ is merely putative – and hopes of such a thing have been dashed every single time they’ve been raised in the past. What might make this one different is that they’ve made its nickname sound like a late-version operating system.

  2. Fun post, Matthew! I only read a few of the articles after seeing the “discovery” headlines. I’m afraid I’m becoming far too cynical in my old age for immediately, I did not believe there was a discovery. I’d like to say that it was my general science background but alas, it was my cynicism. As I mentioned on Facebook, I was waiting for “the truth” from you. As usual, you did not disappoint. Thanks!

    1. Thanks! Yes, I have to say I am fairly cynical myself about anything actually being found – the correlation of just six KBO’s is a slim analysis. Over 450 are known and I’d expect that a large outer ‘shepherd planet’ would affect many more of them. But we’ll see! I guess in many ways the hype of this ‘discovery’ reflects the fact that we are living in a time of incredible science; the world of computing and the side-effects of the hardware technology, such as the CCD, has opened up the universe to us in ways not possible even 20 years ago. I suspect this is where a lot of the hype has come from. Mix that with the ‘instant gratification’ aspect the new generation is conditioned to via the internet and I think there is an expectation that – I am quite certain – won’t be met. Even if Planet 9 is there to be found, as described, it’s going to take a while. But there is always hope. And who knows what else is out there? For the curious, it is an amazing time.

  3. Since it is “shepherding” things and is waaaaaaaay out in an exotic part of the solar system, the name should be Tobe Ra’ah, which has the double benefit of sounding exotic and translating to good shepherd.

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